Drilled Exhaust - Real World Results and Weigh-In On Benefits

[58Evinrude]

What year is that motor?

93 block on a 92 mid.

[Gordon02]

Quote - So are/did you do any other work/mods to motor?

Yes, well sort of. I want to turn it a little higher and want durability. Performance wise, just mild - I am smoothing the exhaust chest and will match the base of the block to the gaskets where the exhaust goes through the adapter plate.

Top-pinned and balanced pistons, rods balanced and beams polished, metal rod bearing cages, and I've got a BP tuner that I was going to use based on the old Bass&Walleye Boats article, but I've got some advice not to run the BP tuner from a very reputable builder. I've got the nice WMV 1.3" carbs and plan to run velocity stacks. I'm not sure what boat this will be put on, I just need to be busy, so I'm building a 2.0! I'd love to find a nice Glastron GT150/160 or perhaps a classic "Cadillac finned" 18~20 foot runabout from the 60's to rebuild. Gordon

rods

[Gordon02]

Does anyone have a photo/image of the 5 holes? I'd like to get an ideal of spacing and location.

Asking a lot here, I know......

Thanks, Gordon

[mrichartz]

Better to ask alot than to screw something up..
Yes with other work your doing drilling will make a difference more than just noise..

[FORBESAUTO]

Don’t have pics handy. But when you remove the liner and look down into mid, the holes are drilled on each side of sealing lip in floor for liner, evenly spaced from front to back, 5 on each side. I use a long drill bit with an extension on it and drill downward from inside. You can make you a little template to lay on each side from wood or something to make it easy to keep em nice and straight.

[FORBESAUTO]

Which mid do you have, the newer fishin style with rubber hose on above exhaust water relief or other style with reliefs in the slots? You can get trick tuning the boot style and not be noticeable externally. When you remove the liner, you’re already deleting one the loops/baffle the exhaust has to go through to reach the upper relief, so it’s gonna help idle. If you still need more relief at idle to help loading up, remove the boot from mid and there is a second wall behind opening that’s part of a channel that forces exhaust to make a loop through adapter plate before exiting. You can drill a hole in that wall to allow exhaust straight out to opening, you a can adjust the size of the hole depending on how mean of an air pump you have to lower back pressure enough to help prevent fuel puddling/ loading up

[Gordon02]

The block/carbs are a 1996 powerhead replacement. Everything else is a 1989 SeaRay 135. I'm going to paint and decal it in Mariner colors - leaving all numbers off of it. I like clearing over the decals once installed using only urethane.

This is the mid section with the exhaust tube removed.

[tux974]

What has also helped in idle once liner is removed even on the O/S mids is drilling a couple of 3/8" holes in the rectangular slots under the adapter plate.

[tlwjkw]

http://www.bbcboards.net/showthread.php?t=20163

[big tom]

So if performance gain is little is there a happy medium?. Like just pulling the liner out?. Don't really want the noise from the twins like I have had with dry pipes on big blocks before.

[Rigaud]

The benefit from exhaust releif is hole shot no H.P. gains.

[big tom]

The benefit from exhaust releif is hole shot no H.P. gains.

What i meant by happy medium was noise level. With twins and motors hanging on back unlike I/O's with exhaust coming out under swim platform. Theres usually people in back seat 6 feet away.

[FORBESAUTO]

No real world test, but I would think removing just the can would help idle and just off idle acceleration performance by removing one loop/baffle the exhaust has to make before reaching above water relief and having a larger area. It will change the tone and be just a tad louder at idle because of this, but no where near obnoxious. Shouldn’t be much difference up and running in noise, but will be a little louder due to exhaust coming out of water drain in lower.

[Rigaud]

What i meant by happy medium was noise level. With twins and motors hanging on back unlike I/O's with exhaust coming out under swim platform. Theres usually people in back seat 6 feet away.

Yes a lot noisier on the back seat, I`m mostly with one passenger along side.

[Rigaud]

No real world test, but I would think removing just the can would help idle and just off idle acceleration performance by removing one loop/baffle the exhaust has to make before reaching above water relief and having a larger area. It will change the tone and be just a tad louder at idle because of this, but no where near obnoxious. Shouldn’t be much difference up and running in noise, but will be a little louder due to exhaust coming out of water drain in lower.

I started with the can removed, no improvement on hole shot. I opened up the mid on my OMC similar to Mercs, 2 holes drilled through 1-1/2" each. Loud yes but the hole shot is awesome. so much that I no longer use a hydraulic jack. Fixed jack.
1987 Valero, 1987 Johnson GT, setback 5.5" plus the built in setback 7". Coned lower LWP, D.A.H. chopper 27p heavy cupping and I'm 1-1/4" above pad. I get up on step easy 2 people and 4 I play with the throttle to get the sweet spot no problem.

That's the reason for the exhaust relief, relieve the back pressure in the water with quick throttle response to get up on the pad instantly. For me.

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